Plastic lenses, which are lightweight and unbreakable compared to inorganic lenses and can be dyed, are increasingly used in optical elements such as spectacle lenses and camera lenses. Resins for making plastic lenses are required to exhibit high performance, such as high refractive index, high Abbe number, low specific gravity, high heat resistance, and the like. Various resin materials for making lenses have been developed and used to date. A representative example thereof is a plastic lens made from a polythiourethane resin (Japanese Unexamined Patent Application Publication Nos. 60-199016, 62-267316, and 63-46213). A polythiourethane resin that exhibits higher refractive index due to an increased sulfur content of thiol used in the polythiourethane has also been developed (Japanese Unexamined Patent Application Publication Nos. 2-270859 and 7-252207).
According to these methods, a high refractive index can be achieved while exhibiting a relatively high Abbe number; however, in order to obtain a polyurethane resin having a yet higher refractive index, a novel thiol having a high sulfur content or a novel polyisocyanate having a high sulfur content must be developed. Despite the need, the development of novel compounds is not easy. Moreover, in order for it to be used in optical elements such as resins for making plastic lenses, physical properties, such as transparency, hue, heat resistance, and shock resistance, other than the refractive index must also be high. Development of a high-refractive-index urethane resin that satisfies all of these requirements is highly difficult.
In a typical conventional process of making a thiourethane resin by polymerization of polythiol and polyiso(thio)cyanate, the polymerizable composition has been prepared such that the molar ratio of a mercapto group to an iso(thio)cyanato group is 1. No examples that investigated the molar ratio and the physical properties of the resin have existed. The reason for this is because when the molar ratio is made greater than 1 using a conventional polythiol compound, i.e., when the mercapto group is excessive relative to the iso(thio)cyanato group, the heat resistance of the resin prepared by polymerization becomes dramatically poor, and the resin may not be applied to optical elements such as plastic lenses. In general, however, high-sulfur-content compounds exhibit high refractive index; hence, the refractive index of polythiol compounds is in most cases higher than that of polyiso(thio)cyanate. Accordingly, if a thiourethane resin can be prepared by using an excessive amount of polythiol, which has a higher refractive index than polyiso(thio)cyanate, a resin with higher refractive index can be made.
Accordingly, an object of the present invention is to provide a method for easily obtaining a high-refractive-index resin.